In my experience, nothing prepares you for the overwhelming reality of a significant experience. A first kiss, the birth of a child, the death of a loved one. One can imagine all these things in advance, often endlessly so. But when they actually happen, theory flies out of the window and the sheer force of the experience takes over.

It is exactly the same in politics. Politicians often pride themselves on their ability to think round corners, to anticipate secondary and tertiary outcomes, and thus to discount the impact of the foreseeable as they move seamlessly on to the next thing. But they deceive themselves too. Even when the long expected occurs in politics, its impact is far greater than the clever people can ever imagine. And I guarantee that it will be like that for Nick Clegg and the Liberal Democrats tomorrow.

"Although I anticipated the total of the losses, somehow when some of the losses happen to you it feels much worse," wrote Richard Crossman in his diary after the first Wilson government was clobbered in the 1965 local elections. "Birmingham, twelve seats lost; Coventry, four seats lost. This is tremendously serious," Crossman continued. "One can't expect the electorate to vote Labour after nearly eight months of Labour government when, despite sowing a lot of seed, we have reaped nothing," he concluded.

Nearly half a century on, that's pretty much where Clegg and the Lib Dems find themselves today. Clegg's clever people have long claimed that this week's local and devolved elections would be bloody but bearable. A month or so ago, however, they began to grasp that the Tories were implacably committed to the defeat of the AV referendum. That's why this week's elections have been the event that broke the coalition's sense of fraternity. But even so, the hurt today will be deeper, more shocking and, above all, more politically consequential than they expected. Crossman's three raw reflections – it feels worse, it is tremendously serious, and we have reaped nothing – will be Clegg's too.

To be fair, there is a measure of pain for all the main parties. The media, ever ready to kick a man when he is down, will concentrate on Clegg. But this is not a good day for the Conservatives. The tide is starting to turn on the Tories in English local government. The party is becalmed in Wales and Scotland. And, for Cameron, even AV is a reminder that behind every silver lining there is still a dark cloud, since both the coalition and the Tory party will now be much harder to manage.

Even Labour has problems. The election of lots of new Labour councillors should not be dismissed. Nor the expected re-election in Wales. But the outcome in Scotland is a humiliation in what used to be Labour heartlands, especially when compared with the hopes so recently. And the very serious Labour splits over AV show how Ed Miliband's writ, like that of the Islamabad government in Pakistan, simply does not run in the tribal areas. If this referendum splits the Labour party as deeply as the European one did in the 1970s – and it could – then today will deliver as much pain as gain.

These elections mark a turning point in British politics in three significant respects. The first, not to be underestimated, is in Scotland. It now seems likely that Alex Salmond's Scottish Nationalists will have a second term at Holyrood. This surely means that there will at last be a referendum on Scottish independence some time before 2016. The default assumption across the political class north and south has always been that this referendum would be lost. But Salmond's re-election might just be a game-changer. If he pulls it off, the political consequences for Scotland and for the remainder of the UK would be massive – not least for the prospect of a future Labour government in London. Take Scotland more seriously.

The second is the effect of the expected rejection of the AV referendum on the prospects for constitutional and electoral reform more generally. Those of us who thought AV would win have to hold our hands up here and confess to getting this wrong. But the lesson of this referendum is twofold: first, that reformers need to prepare their case better, maximise their support better, detach the issue as far as possible from partisan advantage and then avoid entangling the referendum with important elections between the parties; second, it is to recognise that the Tory party is, by and large, reform's enemy,

Those who are against reform, and not just Conservative opponents, will now point to the AV result, like the three-to-one rejection of John Prescott's north-east devolution plan in 2004, as proof that, in the absence of the national grievance that exists in Scotland and Wales, voters will back the status quo. Perhaps. But the lesson of Scottish, Welsh and Northern Irish devolution and, more recently, of the 2011 referendum on Welsh assembly powers, is also that genuine cross-party political preparation is an essential engine of referendum success. Reformers – which hopefully still includes the Labour party in spite of its abject performance on AV – should therefore now establish a UK constitutional reform convention, along the lines pioneered in Scotland 20 years ago.

There is a direct link between that proposal and the third and most immediate effect of May 2011 voting. The referendum campaign has marked the breaking-point of any attempt to turn the Conservative-Lib Dem coalition into a longer-term liberal centre-right project. That does not mean that the coalition was the wrong course for the parties to take a year ago, or that it is now at an end. Neither is true. But it does mean that the coalition will now become a more limited transactional alliance and not the marriage of consenting liberal hearts that some have imagined. The questions of when it ends, how it ends and what happens next are now all on the table, senior Lib Dems now say. To which I would add that the question of Clegg's leadership of his party at the next election is now also there too.

None of this is to say that the rift between Labour and the Lib Dems is about to be healed. Massive uncertainties still litter this part of the political landscape, not least in the destructive form of Labour's infinitely stubborn tribalist tendency. But Labour needs the Lib Dems and vice versa. And the way towards some sort of evolving re-engagement between the progressive parties is beginning to open up. And that, amid so many mixed messages from the voters this week, is a small, flickering still-uncertain sign of hope for the future.